1. Field of the Invention
This invention relates to a window unit having an improved frame sill for mounting on a pre-installed sill disposed in an opening of a building wall.
2. Prior Art
Frame sills of the heretofore-proposed exterior window units shown in U.S. Pat. No. 3,984,954, issued Oct. 12. 1976 generally include a generally horizontally extending base portion having a plurality of parallel horizontally spaced rails or flanges formed on its upper surface for rolling engagement with respective rollers carried by the associated sashes at their lower edges for guiding the horizontal movement of the sashes therealong, and a vertical mounting portion extending upwardly from the horizontal base portion at its inner edge adapted to be mated with and secured to a vertical exterior surface of a pre-installed sill in a building opening. Since a plurality of rails are disposed in parallel side by side relation, the horizontal base portion has a relatively great width, which detracts from the appearance of the frame sill. Even more important is the ability of the horizontal base portion to support the overall load of the sashes. Where the sashes are heavy, the horizontal base portion is susceptible to deformation along a line at which the horizontal base portion and the vertical mounting portion are connected together. To avoid this, it was necessary to reinforce the horizontal base portion. The most common approach was to provide a frame sill in the form of a hollow structure of generally rectangular cross section. This procedure is undesirable, however, because it not only adds to the material cost but also retards the installation of the frame sill. A further difficulty encountered with the conventional frame sills is the problem of preventing rain water from intruding into the interior of a building through the exterior window unit. It was necessary to increase the height of the vertical mounting portion to provide better weather resistance.